= Immunoglobulin G
{title2=$H_2L_2$}
{wiki}
= IgG
{c}
{synonym}
<Immunoglobulin G> is an <antibody> class whose conventional four-chain architecture supplies the example above.
An ordinary <IgG> has two matching <immunoglobulin heavy chains> and two matching <immunoglobulin light chains>, abbreviated $H_2L_2$. Its gamma-type <immunoglobulin heavy chains> contribute both to recognition and to interactions with immune partners.<Antibody Structure and Molecular Immunology by Gerald M. Edelman>[Gerald M. Edelman, Antibody Structure and Molecular Immunology, printed pp. 35–37, Table 1 and Figure 3: gamma chains, four-chain architecture and separate functional regions]{ref}
\Image[/igg-domain-and-disulfide-connectivity.png]
{title=<IgG> chain continuity and <protein disulfide bond> connectivity}
{description=Original chain-connectivity schematic. Gold crossbars denote interchain <protein disulfide bonds>; intrachain crosslinks are omitted. The drawn crossbar count and geometry illustrate connectivity, rather than atomic coordinates or a universal <IgG> subclass inventory.}
A useful way to read this picture is to follow a single <polypeptide chain> continuously from its <N-terminus> to its <C-terminus>, then examine the crosslinks separately. The same <immunoglobulin heavy chain> contributes to a <Fab fragment> and to the <Fc fragment>; these named fragments are not pre-existing separate chains. Conversely, one <Fab fragment> contains material from two different chains. The <protein primary structure> and the fragment inventory therefore answer different questions about the same <protein>.
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